There were some interesting articles on the urinary tract microbiomes in men who have sex with women. Bacteria can live in the low oxygen part of the urinary tract and could be traced between partners. Not STDs, in this case. Women can pass it to men, and vice versa. Interesting area of study. Bacteria are so adaptable!

Edit: found it!

https://www.newscientist.com/article/2365466-the-microbiome-inside-the-penis-can-be-altered-by-vaginal-sex/